…

IF:

Syntax:

if (expression) {
   Statement(s) to be executed if expression is true
}

Example:

let x = 5; 
if(x>4){
    document.write(x)
  }

ELSE:

Syntax:

if (expression) {
   Statement(s) to be executed if expression is true
} else {
   Statement(s) to be executed if expression is false
}

Example:

let x = 5; 
if(x>4){
    document.write(x)
  } else{
document.write("No");
}

ELSE IF:

Syntax:

if (expression 1) {
   Statement(s) to be executed if expression 1 is true
} else if (expression 2) {
   Statement(s) to be executed if expression 2 is true
} else if (expression 3) {
   Statement(s) to be executed if expression 3 is true
} else {
   Statement(s) to be executed if no expression is true
}

Example:

let x = 2; 
if(x>4){
    document.write(x)
  } else if (x>1){
document.write("No");
} else{
document.write("maybe")
}

Switch Case:

Syntax:

switch (expression) {
   case condition 1: statement(s)
   break;

   case condition 2: statement(s)
   break;
   ...

   case condition n: statement(s)
   break;

   default: statement(s)
}

Example:
let a =1
let b= 4
switch (a+b){
case 2: document.write("2")
document.write("2 test")
break;
case 4: document.write("4")
document.write("4 test")
break; 
case 5: 
document.write("5")
document.write("5 test")
break;  
default: document.write("invalid")
}

While Do:

Syntax:

while (expression) {
   Statement(s) to be executed if expression is true
}

Example:

let a=5
while (a>0){
document.write(a+"W")
a--
 }

Do While:

Syntax:

do {
   Statement(s) to be executed if expression is true
}  while (expression) 

Example:

let a=5
do {
document.write(a+"W")
a--
}  while (a>10)

For Loop:

Syntax:

for (initialization; test condition; iteration statement) {
   Statement(s) to be executed if test condition is true
}

Example:

for(i=5; i>0;--i){
document.write(i)
}

For..in (Iterating through object with enumerable properties) :

This should be used only with objects that have elements stored as key value pairs, else if used with normal arrays, the index of the elements will be printed instead

Syntax:

for (variablename in object) {
   statement or block to execute
}

Example:

var test= {a: 1, b: 2, c: 3}; 
var test2= [10,11,15]

for (a in test) {
    document.write(a+"<br />"); 
// will print the key value of the elemnt (output is abc)
}

 for (a in test2) {
    document.write(a+"<br />"); // will print 0,1,2 (index value)
}

For..of (Iterating through object) :

Syntax:

for (variablename of array) {
   statement or block to execute
} 

Example:

var test2= [10,11,15]
 for (a of test2) {
     document.write(a+"
"); // will print 10,11,15 (actual value)
 }

Loop Control:

Continue, Break and label:

Continue:

1. Continue keyword skips all the remaining code and navigates to next iteration
2. Continue can be used only inside iterative blocks eg for, while , do..while , and for in, it cannot be used in switch and normal code block

In below code , document.write will be executed only if a!=1 , if a==1 , then the remaining code is skipped and the for loop goes to the next iteration.

Example:

let test=[1,2,3,4,6];

for (a in test) {

if(a==1){
  continue;
 }
document.write(a+"<br />"); // <br /> indicates next line
 }

Break:

1. Break is used to skip remaining code and iterations
2. Once break is encountered, the code exists from the loop and continues with the outer code

The below code prints only 1 and 2, and on encountering 3, it exists from the loop and prints ‘exited’

Example:

let test=[1,2,3,4,6];

for (a in test) {

if(a==3){
  break;
 }
document.write(a+"<br />"); // <br /> indicates next line
 }
 document.write("exited");

Label:

• Javascript allows controlling the continue and break code flow using labels
• labels are used to identify loops or block and correctly specify which loop to break or continue
• Any non reserved keyword could be used as label

In below code the inner loop is exited on getting a=3 and outer loop is broke when b=”c”

Here, we have labeled inner loop as inner and outer as outer.

Example:

let test=[1,2,3,4,6];
 let test2=["a","b","c","d","e"];
 outer:
 for (b of test2){
 inner:
 for (a of test) { 
 if(a==3){
   break inner;
  }
  if(b=="c"){
   break outer;
  } 
 document.write(a+ "<br />"); //  indicates next line
  }
  document.write(b+ "<br />"); //  indicates next line 
 }
  document.write("exited");

You can use continue the same way to tell the loop to skip remaining lines and goto next iteration of the specified loop.

Example:

let test=[1,2,3,4,6];
  let test2=["a","b","c","d","e"];
  outer:
  for (b of test2){
   document.write(b+  "<br />"); //  indicates next line  
  inner:
  for (a of test) { 
 if(a==3){
    continue outer;
   }
   if(b=="c"){
    break outer;
   } 
  document.write(a+ "<br />"); //  indicates next line 
   }
 }
   document.write("exited");

Label could also be used with block

here we labeled the block ( code between { }) as foo, and in the second line we are existing this block so the third line within the block will not get executed.

foo: {
 document.write('face'+"<br />");
  break foo;
  document.write('this will not be executed');
}
document.write('swap');

Java script simple statements could be ended without semi colon

Statements:

Both the below statements are valid

var a= 6;
var a=5

you could use ‘;’ semicolon to write multiple statement in single line

var1 =10;var 2=23;var 5= 9

Java script is case sensitive, meaning ‘Time’ and ‘time’ are two different elements.

Comments:

//single line comment

/* multi line statement
can be made like this */

html comment closing ‘ → ‘ is not detected by Javascript

Data Types:

Primitive:

JS following primitive data types:

• Numbers — 1,2,3 ( Java script does not have separate representation for floating point and whole numbers, all the numeric are represented as floating points (Eg: 2.0)
• Strings — “name”
• Boolean — true or false

also supports ‘null’ and ‘undefined’

Composite

JS also so support data type called ‘object’

Declaring variables:

Variables are elements that store data and helps in re-usability

In JS , variables needs to be declared before it could be used. In JS , variable declaration could be made mainly using three keywords and are :

1. var
2. let
3. const

Note: JS is dynamically typed (untyped) , meaning data type is detected dynamically. For instance, if for below variables , we store username = 1 and password = “we”; then the data type of username will be taken as Number and password as string.

Using Var

var username
var password

Multiple variable declaration in a single line is also permitted

var username,password,url

Note:

• Variables declared with var are function scoped.
• Could be modified
• Could be re-declared ( It will replace the previous variable)
• Does support hoisting (meaning that , if the variable is called before declaring , then JS will automatically declare and initialize it with value ‘undefined’)

console.log(username)
var username = "user"

// This prints "undefined" because java script changes this code internally as 

var username = undefined
console.log(username)
username = "user"

//This is called hoisting

Using let:

let username;

Note:

• Variables declared with let are block scoped.
• Could be modified
• Could not be re-declared ( It will replace the previous variable)
• Does not support hoisting. Variable should be declared before it could be used

Using const:

const username;

Note:

• Variables declared with const are block scoped.
• Could not be modified
• Could not be re-declared ( It will replace the previous variable)
• Does not support hoisting. Variable should be declared before it could be used

To know more read : ( Read : https://medium.com/@toddhd/javascript-understanding-the-difference-between-var-let-and-const-d0390da913 )

Variable Scope:

JavaScript only two scope :

• Global — Available anywhere in the code
• Local — Available only to the local function in which its defined

Variable naming rules:

• No reserved keyword are allowed : https://www.tutorialspoint.com/javascript/javascript_variables.htm
• Should start with alphabet or underscore

Operators in JavaScript:

• Arithmetic Operators

+ , — , * , / ,% , ++, —

• Comparison Operators

==, !=, > , <, >=, <=,

• Logical (or Relational) Operators

&& (AND) , || (OR), !

• Assignment Operators

=, += (add and assign), -=, *= ,/=, %=

• Conditional (or ternary) Operators

?:

eg: a==b? true:false

• Bitwise Operators